. This proposal focuses on the mechanism of attachment of Pneumocystis carinii (Pc) to alveolar macrophages (AMs). Previous in vitro studies by the primary investigator (PI) and others have provided data indicating that Pc attachment can be mediated by several different mechanisms including fibronectin, vitronectin, mannose receptor, and Fc receptor. However, it is unclear which of these may be the predominant mechanism in vivo. In addition, the mechanism of attachment may be dependent on whether the host is immunocompetent or immunocompromised. Pc attachment, however, may not necessarily trigger effective phagocytosis or killing of the organism. The investigators have proposed the following hypothesis to be tested: Immunosuppression results in Pc pneumonia because of a specific defect in the attachment, phagocytosis or killing of Pc by AMs. In order to test this hypothesis, the investigators will use Pc derived from Pc-infected Scid mice in both in vitro and in vivo assays to examine mechanisms of attachment/phagocytosis and killing of Pc. The in vivo model will involve the study of Pc-AM interaction in normal and CD4 lymphocyte-depleted mice following instillation of FITC-labeled Pc into the airway of mechanically ventilated animals. At defined time points, AMs obtained by bronchoalveolar lavage will be assessed for the attachment and phagocytosis of FITC-Pc organisms by direct fluorometry, confocal laser microscopy and flow cytometry. Thus, in this application, the investigators propose the following specific aims: 1) to determine the relative potency of ligands (SP-A, Fn, Vn, or Ig) in the mediation of attachment/phagocytosis of Pc or liposomses incorporated with specific the proteins by AMs from normal mice or by AMs from CD4 lymphocyte-depleted mice; 2) to determine which of the ligand mediated mechanisms of Pc attachment/phagocytosis trigger a cytotoxic response by AMs and to determine the mechanisms of Pc killing by AMs; 3) to determine if cytokine priming of AMs from normal or CD4 lypmphocyte-depleted mice augments and/or restores normal attachment/phagocytosis/killing of Pc bound to AMs by specific ligand- dependent mechanisms; 4) to determine in vivo the mechanisms of Pc attachment/phagocytosis/killing by normal AMs; and 5) to determine in vivo the mechanisms (or absence of mechanisms) of Pc attachment/ phagocytosis/killing by AMs from CD4 lymphocyte depleted mice.